1. Technical Field
The present disclosure relates to light emitting devices, and more particularly to a light emitting device that includes a light emitting element configured to emit a blue light, and a sulfide phosphor configured to emit a green light upon absorbing a portion of the blue light emitted from the light emitting element.
2. Description of the Related Art
There have been known light emitting devices adapted to emit a white light. This kind of light emitting device includes a light emitting element to emit a blue light, a green phosphor to emit a green light (or a yellow-green phosphor to emit a yellow-green light) upon absorbing a portion of the blue light emitted from the light emitting element, and a red phosphor to emit a red light upon absorbing a portion of the blue light emitted from the light emitting element. Such light emitting devices adapted to emit a white light are used in various applications, such as illumination devices and backlights for various displays, such as liquid crystal displays.
In recent years, light emitting devices having all or a portion of such phosphors replaced by a sulfide phosphor have been developed. For example, JP 2014-024918 A discloses a white light emitting device that includes a green sulfide phosphor and a red phosphor.
Green sulfide phosphors have high light emitting efficiency and a narrower full width at half maximum of the emission spectrum compared to that of a β-sialon phosphor that is commonly used as a green phosphor. Thus, the light emitting device using a sulfide phosphor has an advantage of a wide color reproducibility range when combined with a color filter of a liquid crystal display or the like. Further, matching the peak wavelength of the color filter (a wavelength at which its transmittance reaches a peak) to the emission peak of the sulfide phosphor allows for more light to pass through the color filter, which improves the light extraction efficiency with less attenuation of the light in use of the color filter. Particularly, the green sulfide phosphors have higher light emitting efficiency than that of conventional green phosphors, allowing for liquid crystal panels of high brightness.
However, these conventional light emitting devices employing such a sulfide phosphor are designed to use a phosphor such as CaS:Eu, (BaSr)3SiO5:Eu, or the like, as the red phosphor, which may lead to the occurrence of secondary absorption. That is, a portion of the green light (or the yellow-green light) emitted from the green sulfide phosphor that has absorbed the blue light is absorbed by the red phosphor which then emits a red light. The occurrence of such secondary absorption leads to a reduction in the luminous efficiency of the whole light emitting device. On the other hand, in many applications such as in displays and illumination devices, there has arisen a need for a light emitting device that can emit brighter light with lower power consumption, that is, which has high luminance efficiency.